Regular expression for matching time in military (24 hour) format

This should do it:

^([01]\d|2[0-3]):?([0-5]\d)$

The expression reads:

^        Start of string (anchor)
(        begin capturing group
  [01]   a "0" or "1"
  \d     any digit
 |       or
  2[0-3] "2" followed by a character between 0 and 3 inclusive
)        end capturing group
:?       optional colon
(        start capturing
  [0-5]  character between 0 and 5
  \d     digit
)        end group
$        end of string anchor